How to connect slack channel with mule?

As @phulme pointed out, there is a Slack connector which can be installed from the Exchange. Check the link in Phil's post. Also, there is a demo project in the connector's source code on GitHub: https://github.com/mulesoft/slack-connector/tree/develop/demo/slack-demo-3.6

And finally, here's an (somewhat old) article on the Slack connector: https://dzone.com/articles/slack-connector-in-mule-esb

A bit of Googling can be helpful...